Silver nitrate of high purity has many known uses. It is required for the manufacture of photographic materials, pharmaceuticals and catalysts. Silver nitrate is prepared by reacting metallic silver with nitric acid. The impurities in metallic silver remain in the resulting silver nitrate solution. The impurities include metals such as copper, iron, lead, nickel, tin, bismuth, zinc, chromium, manganese, antimony, cadmium, halides, phosphates, arsenates, selenium, etc.
Silver nitrate solution is often purified by batch treatment of the solution with silver oxide in agitated tanks for at least 1 to 8 hours. Flocculating agents are frequently used in the treatment. The batch is sampled and the pH measured. A pH of 6.0-6.2 was deemed acceptable to promote precipitation of impurities. Following this treatment period, the silver oxide solids are removed, along with precipitated and adsorbed impurities, typically by a gravity settling and filtration.
The reliability of pH measurement in concentrated silver nitrate solutions is quite poor. Dehydration of the pH electrode and fouling of the reference electrode cause pH measurement drift that prevents its use as a continuous measurement device.
The main disadvantage of batch methods is the high in-process inventory of materials required because of the long treatment times involves. Since a high inventory is required, batch purification is limited to use only in bypass or recycle streams in conjunction with multiple stages of crystallization to comprise a total process to produce photographic quality silver nitrate. Batch processes also require relatively large batch tanks.